The invention relates to a wrap-around drive comprising a circulating belt means, which is driven by at least one drive element and which drives at least one driven element, as well as at least one first tensioning device acting on the belt means in a region of the wrap-around drive, in which this belt means leaves the drive element in the circulating direction and reaches the closest driven element, and at least one second device in a region of the wrap-around drive, in which the belt means reaches the drive element in its circulating direction and leaves the closest driven element.
Such wrap-around drives are used, for example, for timing and/or accessory drives in internal combustion engines. The traction means, for example, the timing belt, is here driven by a driving gear mounted on a crankshaft of the engine and drives driven gears, which are connected to timing shafts or camshafts of the engine. For limiting transverse vibrations in the timing belt, this belt is led over a guide on its tensioned side running into the driving gear and a force tensioning the belt is applied by a tensioning device to the slack side of the belt running out from the driving gear.
EP 1 262 685 also shows a wrap-around drive according to the class, in which for limiting transverse vibrations of a timing belt, a force is applied to this belt both on its tensioned side running into the driving gear and also on its slack side running out from the driving gear. The forces are set here by a rotating, adjustable ring body, to which guide rails are attached that act on the timing belt. The tensioning force acting on the timing belt increases or decreases with the degree of rotation of the ring body. The rotation of the ring body itself is generated by a combination of oil pressure and spring force.
DE 196 16 081 C1 shows a wrap-around drive comprised of belt disks and an endless belt with a device for steadying belt vibrations, in which a guide plate is arranged fixed at a close distance to the belt for reducing belt vibrations in a corresponding critical region.
Limiting transverse vibrations in wrap-around drives is of great importance with respect to their functionality, service life, and noise output. This applies independent of how many driving and driven gears are actually used for a wrap-around drive. The belt means can involve, for example, a chain or a toothed belt of a timing drive, which synchronizes crankshafts and camshafts with each other, or, for example, also a driving belt, which connects a belt disk of a drive shaft to the belt disk of a driven shaft for some assembly in a driving manner.
For transverse vibrations that become too large, adjacent components can be damaged, if a toothed belt or a chain temporarily loses positive-fit contact with a driving or also driven element due to transverse vibrations that are too large. Furthermore, unsuitably high mechanical loads for the belt means itself can occur due to excessive transverse vibrations, which lead to a shortened service life of the belt means. In addition, excessive transverse vibrations cause a comparatively high noise generation.
Finally, synchronization errors between at least two camshafts and/or between these and the crankshaft of an internal combustion engine can occur, when the wrap-around drive is lengthened due to wear and a tensioning device on a belt means strand compensates this by an increased tensioning path. Because this single tensioning device is typically activated by an actuator acted upon by the oil pressure of the internal combustion engine, in particular, at the start of the internal combustion engine, insufficient oil pressure is present in the actuator, so that disadvantageously, tooth jumping is hard to prevent in known wrap-around drives.
To be noted is also the increasing complexity of accessories for usually only limited space relationships in the region of the wrap-around drive, as well as the necessary flexibility of the wrap-around drive due to the increasing number of different accessories with respect to adaptability to different operating conditions.